1. Field of Invention
This invention relates to variable frequency oscillators (VFOs) for use in applications requiring rapid starting, such as in data recovery from magnetic disk recording systems. In addition, such oscillators may be used in data transmission systems which also require the use of tracking oscillators. A particular application of this oscillator device is in the tracking of non-self-clocking data, i.e., data where there is a priori knowledge of the transmission rules based upon which the data can be correctly interpreted.
Where a VFO is used to track data, a certain amount of time is required in order for the VFO to lock to the raw data. The amount of time required is determined by the bandwidth of the VFO feedback control loop, which is largely a function of the time constant of the control loop. During the transition, or lock-in, period, data cannot be accurately read. Therefore, it is imperative that the transition time be minimized so that the VFO can be rapidly locked to the incoming data.
In some applications, for example in the area of phase locked loop detection, it is typical to provide a free-running VFO having a relatively wide bandwidth. The wide bandwidth enables the phase locked loop to more easily lock onto the desired frequency and phase of incoming data. However, increasing the bandwith of the phase locked loop circuit renders the phase locked loop more sensitive to random noise in the data stream, reducing the ability of the phase locked loop to decode accurately incoming data. In order to minimize the random noise it is desirable to decrease the loop bandwidth. However, relatively narrow band phase locked loops using free-running oscillators are subject to false lock conditions. Once the loop is in the false lock condition it cannot recover by itself. Various schemes have been devised which provide a wide bandwidth during the lock-in interval and a narrow bandwidth during the tracking interval. However, such schemes are subject to other undesirable limitations such as undesirable phase transient conditions. What is therefore needed is a variable frequency oscillator which can be stopped and subsequently started in phase with incoming data and which can be employed in a narrow band phase locked loop to take advantage of the improved performance inherent in a narrow-band tracking circuit.
2. Description of the Prior Art
Various variable frequency oscillator circuits are known in the prior art, some of which are described in U.S. Pat. No. 3,810,234 issued in the name of the present inventor. Of particular interest are LC type oscillator circuits which are useful in data recovery systems. Such oscillators as described in the aforementioned patent have now been incorporated in a number of data recovery systems including the Memorex 670 series high speed disk drives. Other versions are found in the Shugart SA-4000 series high speed disk drives, as well the Seagate ST-506 and ST-412 series data storage and recovery systems. The aforementioned patent describes an injection lock oscillator which required a stream of data pulses at start up as a source of synchronization. The Shugart and Seagate devices have related improved technology.
LC oscillator circuits in general are known to provide excellent stability when used in high-noise environments. The energy associated with the oscillations in an LC circuit provide a "flywheel" effect which tends to maintain a VFO operating in phase despite small perturbations caused by transient noise. In addition, LC oscillators are generally capable of performing at frequencies much higher than conventional prior art RC oscillators.
However, known prior art LC oscillators as well as other oscillators of the prior art have experienced problems in starting and stopping because of the finite time required to build up oscillation within the tuned circuit, including problems evident as frequency error during the startup segment.
An oscillator circuit used in a Shugart Associates Series SA-4000 disk drive is of particular interest because it is of the rapid startup type. In the Shugart circuit, a voltage controlled capacitor is employed to adjust the frequency of oscillation of a Colpitts-type oscillator in which feedback energy is introduced across a charge storage device. A voltage controlled capacitor (Varicap) forms a portion of another charge storing device and is used to adjust the frequency of oscillation of the circuit. The Shugart-type oscillator circuit has been found to be subject to uncontrolled frequency error during startup. The amount of frequency error is component dependent and thus varies from device to device in a practical system.
Referring to FIG. 1, a diagram of a prior art oscillator 10 is shown. The oscillator 10 is a Colpitts-type oscillator with a resonant tank circuit consisting of capacitors C1, C2, C3, voltage variable capacitor VVC1 and inductor L1. The conditions necessary for oscillation are provided by 180-degree phase reversal through a NAND gate 12 and a 180-degree phase reversal at resonance across the inductor L1. A transistor 14 in an emitter follower configuration couples a signal at high impedance from a node 16 to the relatively low impedance first input 18 of the NAND gate 12. A second input 20 of the NAND gate 12 is employed for injecting a control circuit for starting and stopping the oscillator 10.
The frequency of oscillation of the oscillator 10 is controlled by an error signal applied at an input node 22 which controls the bias across the voltage-variable capacitor VVC1. As a consequence of error voltage variation, the capacitance of VVC1 is varied, thereby changing the steady state oscillation frequency of the tank circuit according to well known principles.
One of the features of the oscillator 10 of the prior art is a mechanism for very rapid shutdown. For this purpose, a diode D1 is coupled between node 16 through a resistor R2 to the control input 20, and a load resistor R1 is coupled between the output of NAND gate 12 to the common node of inductor L1 and capacitor C1 in the tank circuit thereby to form a current loop which dissipates energy in the tank circuit when the diode D1 is forward biased.
However, the oscillator 10 is subject to undesired frequency variation during the transition following startup of oscillation. This condition is affected by the impedance of logic 1 level of the NAND gate output, the impedance of the base emitter junction in the transistor 14 and the impedance of the forward biased diode D1. The impedance values for resistors R1 and R2 are preferably chosen to provide a DC voltage at the node 16 which is the same as the average DC voltage during steady state operation of the oscillator 10. Any difference in the voltages between the OFF state and the ON state causes an undesired frequency transient during the transition between the states. In practice, it is very difficult to provide matched impedances in this circuit to assure the required condition at node 16. One of the significant problems is the undefined voltage at the logic 1 level of the NAND gate 12 inherent in practical NAND gates.
An intended feature of the oscillator 10 of the prior art is its ability to start rapidly. During the prestart condition, current is maintained in L1 through the current loop including forward-biased diode D1. At startup, the current through L1 is shut off causing the creation of a magnetic field which is useful in rapidly starting the oscillation of the resonant tank. Notwithstanding the features of the prior art oscillator 10, the inherent disadvantages suggest that improvements must be made to achieve more desirable results, such as stable frequency operation during the transition period following startup.
What is needed is a oscillator device which is capable of being started and stopped rapidly and of running at relatively high frequencies, on the order of 100 Megahertz and above, for use in data tracking applications. What is further needed is an oscillator in which frequency perturbations caused by startup transients is minimized. Still further, what is needed is an oscillator circuit which is substantially insensitive to component tolerances.